To make it even, I'll chose a Middle-earth character comparable to their Marvel counterpart.

Captain America - Fingolfin (I'm sure he could do some damage with his skills) Iron Man - Gandalf The Hulk - I'd choose Huan, simply for his brute strength (or Beorn might be good) Black Widow - I would say Melian, or Luthien. But maybe Arwen would be a better match? Hawkeye - Beleg (because he was considered to be the best bowman of his time) Thor - Turin

As the goal is a balanced match, I would go with Arwen as you put in parentheses, over Melian or Luthen, either of whom would wipe the floor with Black Widow I'm pretty sure.

Interesting that you paired iron Man with Gandalf, and Thor with Turin. If I did anything to edit that, i might switch those two pairs. Gandalf and Thor seem to share some similar lightning and thunder powers (like when Gandalf drew lightning from the sky to charge up Glamdring, during the Balrog fight at the beginning of TTT, for example).

For Hulk I was originally thinking Beorn, but Huan might even be better. I was originally thinking Legolas for Hawkeye but Beleg might be an improvement there.

Now, why don't we add to the challenge a little, for a bit of "extra credit": same question, but instead of just one line-up, now come up with four line-ups, one for each of the following time periods:

(1) First Age only; (2) Second Age only; (3) Third Age during the time of TH only, and (4) Third Age during the time of LOTR only.

Preferrably, a line up of characters that would have co-existed according to Tolkien's writings.

I'll try to come up with my own 4 line-ups in another reply post. I still intend to come up with a reply to my OP with my "dream team" line-up, though I don't think it'll be better than yours. A bag is like a hole that you can carry with you.

The Hulk........Gothmog the Balrog. Or Ancalagon the Black. Huan would be good too. I almost went with Beorn here, but I'm not sure he'd be on par with the Hulk at his angry best.

Black Widow....... Arwen.

(I was almost going with Eowyn here, but Black Widow has firearms, thus has a clear range advantage. At best, Eowyn might be able to throw a spear at her, but that's still quite a disadvantage. Also Black Widow has more moves for close-combat, and could probably disarm Eowyn of her sword using her close quarters combat martial arts techniques (CQC).

Arwen balances that out with her water magic, plus she's probably a decent shot with a bow and would probably bring one to the fight. Given her Elven reflexes and speed, I'm not sure Black Widow would be able to disarm her of Hadhafang using CQC.)

Hawkeye.........Legolas (Beleg might be too much for Hawkeye? while Legolas seems just right?)

Thor..........Gandalf (imagine the lightning show for this match-up!) he may have to be allowed to take Maia form for this duel, though. I was thinking Sauron (in armored, mace-wielding FOTR-prologue form) but I think Sauron, with the Ring, would be too much for Thor. A bag is like a hole that you can carry with you.

Not sure about the pairing of Turin and Thor. Could a mortal man stand against the God of Thunder? Granted, Turin is prophesied to be the one who slays Morgoth in the Dagor Dagorath but I always assumed that he'd strike the final blow after the Valar had softened Morgoth up. Does Turin have the power to slay gods and angelic beings on his own?

Captain America - Aragorn Iron Man - The Witch-king (in his armored form, of course) The Hulk - Durin's Bane Black Widow - Eowyn (also considered Faramir. The two of them stalking each other through the forests of Ithilien would be pretty awesome) Hawkeye - Legolas Thor - Gandalf

I would definitely pay to see these matchups! I see nothing to improve upon here. Thor-- Tulkas is an improvement on Thor-- Turin. Iron Man v Dain Ironfoot was an inspired choice. (And was that an intended pun? Iron Man v. Iron Foot? Ha!). Black Widow v Elendil was the most surprising, but it could work. Well done! A bag is like a hole that you can carry with you.

Yeah, the Iron Man/Iron Foot matchup was intentional. But I also figured Dain would be in full armor and a hardy warrior so he'd fare better against Iron Man than others.

As for Black Widow/Elendil, I had the same problem DanielLB did. There aren't as many characters to choose from in the Second Age. I thought about Erendis but she wasn't really a warrior. Plus, who says Black Widow always has to fight the female of the team? I figured Black Widow would be a better fighter than Hawkeye, given her KGB and SHIELD training, so I paired her with Elendil, who was good enough to take on Sauron.

Hulk is insanely durable, and only gets stronger, I suspect Glaurung would have the advantage to begin with, but a enraged hulk would bash the dragons head in.

Iron Man packs a lot of power and guns, Smaug might shack of a few of his blasts but I can not see the golden dragon surviving against the unibeam. Would be quite a exciting aerial battle.

Captain America would need to use all of his skills and shield to overcome the giant bear which is Beorn.

Black Widow was a hard one, her guns and reflexes would put down most opponents. Characters which would survive her bullets are typically extremely powerful beings and way out of her league. I think Shelob is a good choice, two female spiders going at it. Black Widows bullets would certainly hurt the giant spider but she would need all of her skills to avoid the webs and fangs.

Hawkeye: His exploding arrows are powerful against most characters and places him in a league above all of Tolkiens archers. With brains I think Hawkeye should be able to overcome the Balrog.

You're right about the explosive arrows. I wonder if the Elves have any special arrows that would level that playing field. I imagine there are enchantments that could do the job. In the games, like EA's Battle for Middle Earth series of RTS games, the Elves have an arrow upgrade that gave each shot an explosive knock-down effect upon impact, although that's not canon. A bag is like a hole that you can carry with you.

Black Widow vs. Ungoliant? There it is: dwarves are not heroes, but calculating folk with a great idea of the value of money; some are tricky and treacherous and pretty bad lots; some are not, but are decent enough people like Thorin and Company, if you don't expect too much.

Ungoliant, especially once empowered by consuming the two trees overpowered Morgoth. If Black Widow was allowed all of her comic equipment she might get a rare win against Ungoliant. But I personally feel Ungoliant would be right out of her league.

Ungoliant could be a good opponent for Thor or maybe Iron man. Iron mans sensors would allow him to see through her shadows. Her shadow would likely cause the hulk to get lost, and these even a outside chance that she would be able to feed of his gamma radiation concerting him back to Bruce Banner.